1. Field of the Invention
This disclosure is related to a transmission line for down-hole tools such as are associated with drill pipes in a tool string. More particularly, this disclosure relates to a semi-rigid transmission line that is capable of withstanding the tensile stresses, dynamic accelerations, and gravitational loads experienced by the downhole tools when drilling an oil, gas, or geothermal well.
2. Description of the Related Art
The transmission line of this disclosure is provided by placing the various components of the transmission line in sufficient contact with each other that independent motion between them is abated during use.
It has long been the unrealized goal of the drilling and subterranean excavation industries to achieve a real time, high data rate transmission of information from the excavation tool to the surface control systems. For example, in drilling wells, an information stream traveling to and from the drill bit would aid the driller in determining the condition of the drill bit, the nature of the formations being drilled, hazardous conditions developing in the formation and drill string, the condition of the drill string in general, and aid the driller in sending commands to the drill bit and related downhole equipment in order to steer the bit in the direction desired. An important element of such a real time network is a high-speed transmission line.
Transmission lines consisting of wire and coaxial cable have generally been proposed in prior disclosures. Coaxial systems are preferred for their utility and potential for transmitting a signal at high data rates. A coaxial cable is usually comprised of an inner conductive member, a dielectric region, and an outer conductor. Often the cable is encased within a jacket for ease of handling and as an extra measure of protection during use. The inner and outer components are usually comprised of conductive metal. Copper, aluminum, brass, gold, and silver, or combinations thereof, are the preferred materials that make up the conductors. Higher strength materials, such as steel, stainless steel, beryllium copper, Inconel, tungsten, chrome, nickel, titanium, magnesium, palladium, etc., and combinations thereof, have also been used for these components.
Theoretically, the most efficient dielectric region would consist of a gas having a dielectric constant of about 1.0. The dielectric constant of the materials used in the dielectric region is inversely related to the rate of signal propagation along the cable, e.g., the lower the constant, the higher the rate of signal transmission. But an exclusively gaseous system is impractical since in it there would be no means of maintaining the concentricity of the center conductor. Therefore, dielectric materials having low dielectric constants such as polymers and ceramics have been proposed for use in the dielectric region. A substantially porous dielectric may be preferred over a substantially non-porous dielectric in some applications because of its likelihood of increasing the gaseous content of the dielectric, thereby lowering the dielectric constant of the region and increasing the potential velocity of signal propagation along the length of the transmission line.
U.S. Pat. No. 2,437,482 incorporated by reference herein for all it discloses, to Salisbury, discloses the use of insulating beads is taught and a method is provided for configuring the inner and outer conductors to overcome the effects of the beads on signal propagation. U.S. Pat. No. 4,161,704 incorporated by reference herein for all it discloses, to Schafer, shows a transmission line is provided having electronic circuit components such as filters encapsulated therein. The disclosure also teaches the use of fluoropolymer foam dielectric materials such as Teflon®. This disclosure also teaches that in the process of manufacturing the cable, the outer conductor and dielectric region are mechanically reduced by drawing them through a die so as to contact each other and the center conductor. U.S. Pat. No. 4,340,773 incorporated by reference herein for all it discloses, to Perresult, discloses a small diameter dielectric system composed of a first layer of cellular polyparabanic acid that provides a skin surrounding the inner conductor. A second layer of a crosslinkable polymeric lacquer provides a skin enclosing the first layer. In this manner a strong, micro-diameter cable may be produced. U.S. Pat. No. 5,946,798 incorporated by reference herein for all it discloses, to Buluschek, provides for a method of manufacturing the core of the coaxial transmission line. A strip of conductive materials is shaped into a tube and then welded along its seam. After welding the tube undergoes a calibrations step to shape the core into a circular cross section.
In downhole applications, methods have been disclosed for providing electrical conductors along the length drill pipe and other tools. Coaxial transmission line cables have been recommended as the preferred conductor and an integral component for any system seeking to achieve high data rate transmission. The following are exemplary disclosures of these suggested applications.
U.S. Pat. No. 2,379,800 incorporated by reference herein for all it discloses, to Hare, discloses the use of a protective shield for conductors and coils running along the length of the drill pipe. The shield served to protect the conductors from abrasion that would be caused by the drilling fluid and other materials passing through the bore of the drill pipe.
U.S. Pat. No. 4,095,865 incorporated by reference herein for all it discloses, to Denison et al. discloses an improved drill pipe for sending an electrical signal along the drill string. The improvement comprised putting the conductor wire in a spiral conduit sprung against the inside bore wall of the pipe. The conduit served to protect the conductor and provided an annular space within the bore for the passage of drilling tools.
U.S. Pat. No. 4,445,734 incorporated by reference herein for all it discloses, to Cunningham, teaches an electrical conductor or wire segment imbedded within the wall of the liner, which secures the conductor to the pipe wall and protects the conductor from abrasion and contamination caused by the circulating drilling fluid. The liner of the reference was composed of an elastomeric, dielectric material that is bonded to the inner wall of the drill pipe.
U.S. Pat. No. 4,924,949 incorporated by reference herein for all it discloses, to Curlett, discloses a system of conduits along the pipe wall. The conduits are useful for conveying electrical conductors and fluids to and from the surface during the drilling operation.
U.S. Pat. No. 6,392,317 incorporated by reference herein for all it discloses, to Hall, et al., the applicants of the present disclosure, discloses an annular wire harness incorporating a coaxial transmission line connected to one or more rings for use in transmitting high-speed data along a drill string. The coaxial transmission line is connected to the rings that comprise a means for inductively coupling segmented drilling tools that make up the drill string.
In order to make a downhole transmission line practical, the cable of the transmission line must be able to withstand the dynamic conditions of downhole drilling. The transmission line cables that have been proposed in the art have not provided for the harsh environment that will be encountered downhole. Therefore, it is the object of this invention to provide a transmission line cable that can reliably deliver high data rate transmission in a downhole environment where high tensile stresses, rapid accelerations, and high, intermittent gravitational loads are present.